1. Field of the Invention
The present invention relates to a food system for recovering the heat and preventing pollution from waste hot air after nixtamalized corn dehydration and, more particularly, it relates to a process improvement involving heat recovery from the waste hot air laden with particulate produced after flash drying, especially milled corn and the like, and for minimizing particulate emission in both the exhaust vented to the atmosphere and the hot water recycled to the process.
2. Description of Related Art
Nixtamalized corn flour (NCF) is conventionally produced by alkaline cooking of food-grade corn under atmospheric pressure, steeping and washing, grinding the partially cooked corn (nixtamal) and drying to give corn masa flour. At the industrial or commercial level, xe2x80x9cthe milling and dehydrationxe2x80x9d steps are major cost factors. In any case, the availability of an instant flour has many advantages such as shelf-life (4 to 8 months), less labor, lower water and energy use for the customer. Any method which may decrease both time and cost, and still yield an acceptable nixtamalized corn or masa flour product per unit of raw corn, would be advantageous to the final customer.
By combining energy efficiency and pollution prevention technologies is an opportunity for alignment among business, environmental advocates and regulators. The efficient use of resources is key to a sustainable development and industrial ecology. Energy recovery and renewable energy have supplied more than 80% in the US incremental energy requirements since 1973 ($0.25 USD/MM-Btu). But given today""s low prices for natural gas ($1.5-2.2 USD in 1997 and $3 USD by mid-2001: oilnergy.com), no realistic reductions in carbon emissions will happen without concerted national programs and incentives to encourage the faster adoption of efficient and renewable energy as well as natural gas. The success and cost effectiveness of this approach have been proven by redesigning or improving processes: reduce/recycle/re-sell waste, reduce energy use and emissions (Acee, 1997).
In this connection, reference is made to the following U.S. Pat. Nos. 4,594,260, 5,176,931, 5,532,013, 6,025,011 and 6,265,013 requiring a low-temperature drying. However, U.S. Pat. Nos. 4,513,018, 5,558,898 and 6,068,873 only used a high-temperature short time dehydration without a heat recovery system. These prior art methods for the industrial production of corn and masa flour involve accelerated cooking with reduced amounts of water along with short processing times as well as low energy requirements for a high yield of the end product.
As previously mentioned by Rubio in U.S. Pat. No. 4,513,018 and more recently by Sunderland in U.S. Pat. No. 5,558,898, the waste hot air produced by combustion of natural gas with excess air is actually not pure combustion gas, but rather a mixture of hot air, water and particulate matter (very fine particles). This hot exhaust mix is drawn from the dryer by a fan and fed into a cyclone for removal of solid and liquid matter (hereafter referred to as dust or particulate) from such waste hot air for pollution control. A single-cyclone is the most widely used of the dry separator having a medium collection efficiency (80-95% for 15-50 micron size), a low pressure drop (2-5 in. water) and a high throughput (Theodore and Buonicore, 1976).
Another method of treating contaminated air from a food processing operation in the past has been by direct incineration or wet scrubbing. A scrubber is a system which uses a liquid, usually water, to achieve the removal of particulates from a gas stream. Scrubbers (particulate collectors) may be loosely categorized by pressure drop (inches of water) or energy consumption (Theodore and Buonicore, 1976): a) Spray chambers and towers (Knapp, 2000: U.S. Pat. No. 6,019,818), for example, provide the lowest drop ( less than 5 in.) and, correspondingly, the lowest collection efficiencies (70-90% for  greater than 10 micron), b) Centrifugal fan, atomizing and packed-bed scrubbers have a medium drop (5-15 in.) and medium efficiency (90-95%), and c) Venturi-type has the highest drop ( greater than 15 in.) along with the highest efficiency ( greater than 99%). In this connection, a high-pressure venturi scrubber (40 in. water) was used commercially in treating a medical waste incinerator exhaust gas which not only claimed a 44% energy recovery for preheating incinerator or combustion air, but also complied with particulate EPA-limits (Andersen, 2001).
A few food processing plants have implemented both energy recovery and particulate emission using wet scrubbers to improve process efficiency during dehydration:
For example, West in U.S. Pat. No. 3,805,686, describes a batch scrubber system for treating moist air laden with particulate matter from a food smoke and drying chamber, cooled to condense liquid and solid out of the air in a dehumidifying tower, and reheated for recycling to the smoke chamber without discharge to the atmosphere.
Furthermore, Williams in U.S. Pat. No. 6,019,819, discloses another improved scrubber for extracting heat from contaminated waste steam. Waste gas is ducted from a food fryer to a condensing tower by spraying water into the gas. This cooled waste gas is pulled into a low pressure water washer, and clean gas is sucked by a fan and vented.
Many applications for heat recovery included waste hot gas from a high-temperature process furnace, an incinerator and a high-temperature direct-fired dryer. Although the above described prior art methods are capable in improving energy or particulate collection efficiency, a low-cost industrial application was still unavailable in the market at the time of the present invention.
The purpose of this invention is to provide a process improvement by reducing particulate matter and extracting heat and from the waste hot air of a flash dryer and recycle said heat into the furnace and washer as well.
Another object of the invention is to use an industrial method not only with a low-energy trapping and scrubbing apparatus, but also with a food process energy saving.
The above and other objects and advantages of the invention are achieved through a process improvement and apparatus applied to the production of nixtamalized corn flour, embodiments of which include pre-cooking with a lime solution so as to effect corn partial cooking, pre-cooking and washing with reduced energy, stabilizing moisture content to an optimum level for grinding, milling and flash dehydration of the milled corn with reduced energy and particulate emission, dust trapping of a portion of waste hot air which is reused for preheating combustion air, wet scrubbing of the remaining waste hot air where heated water is reused and exhaust air vented, cooling and drying the dry-milled particles, separating and recovering the fine grind so produced from the coarse grind while the latter is further aspirated to remove a hull fraction as corn waste, re-milling the isolated coarse grind and further sieving it to obtain a corn flour for tortilla and derivatives thereof.